wire length, clarification
To: mail11:;-at-cimcad.enet.dec-dot-com (-at-teslatech)
Subject: wire length, clarification
From: I am the NRA <pierson-at-cimcad.enet.dec-dot-com>
Date: Mon, 28 Nov 94 21:22:06 EST
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>> Given a bunch of 22 ga. magnet wire and a 4" thin wall PVC pipe for
>> a form, design a coil that works well. I plan on figuring out
>> if the wire diameter will allow between 400 and 1000 turns and
>> produce a coil with a height to width ratio of about 4:1.
>> Then, calculate this length, and figure out the 1/4 wave frequency
>> based on this length.
> How? Its NOT the same as the length of the wire. The wire length will
> turn out to be _roughly_ twice the 1/4 wave, for most coils. That is,
> 100 meters of wire will resonate at 50 meters. ROUGHLY. Ther is cut
> and try here.
On rereading, i find myself unclear in the above. Any frequency has an
associated wave length. Roughly, a single isolated conductor
(1/4 wave over gorund, in radio talk...) will resonate at the point
where it is 1/4 wave long. That is: a one meter wire will be resonant
at 4 meters. (call it 70 MHz, roughly...). But. Take that same wire
and wind it onto a single layer coil. It Now "looks" "longer". It will
now resonate at (roughly) 2 meters (call it 144 MHz...8)>>). The exact
frequency will depend on the length to width ratio, turn spacing, etc.
For tall skinny coils, it tends to 2-1. For shorter fatter coils it
tends back towards to 1-1 but does not get there. The math is not
(The 1/4 wave wire will ALSO resonate at 144, in a different mode,
and 288, and......)